The Vitamin D metabolites known as 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 (collectively referred to as “25-hydroxyvitamin D”) are Vitamin D prohormones that contribute to the maintenance of adequate levels of Vitamin D hormones, calcium and phosphorus in the bloodstream. The prohormone 25-hydroxyvitamin D2 is produced from Vitamin D2 (ergocalciferol), and 25-hydroxyvitamin D3 (calcifediol) is produced from Vitamin D3 (cholecalciferol), primarily by one or more enzymes located in the liver. The two prohormones also can be produced outside of the liver from Vitamin D2 and Vitamin D3 (collectively referred to as “Vitamin D”) in certain cells, such as enterocytes, which contain enzymes identical or similar to those found in the liver.
The Vitamin D prohormones are further metabolized in the kidneys by the 1α-hydroxylase enzyme CYP27B1 into potent hormones. The prohormone 25-hydroxyvitamin D2 is metabolized into a hormone known as 1α,25-dihydroxyvitamin D2 (ercalcitriol); likewise, 25-hydroxyvitamin D3 is metabolized into 1α,25-dihydroxyvitamin D3 (calcitriol). Production of these hormones from the prohormones also can occur outside of the kidney in cells which contain the required enzyme(s).
The Vitamin D hormones have essential roles in human health which are mediated by intracellular Vitamin D receptors (VDR). The Vitamin D hormones participate in the regulation of cellular differentiation and growth, parathyroid hormone (PTH) secretion by the parathyroid glands, and normal bone formation and metabolism. In particular, the Vitamin D hormones regulate blood calcium levels by controlling the absorption of dietary calcium and phosphorus by the small intestine and the reabsorption of calcium by the kidneys. Under normal conditions, actions of Vitamin D on stimulating intestinal calcium absorption predominate, such that dietary calcium is the main source of serum calcium. However if dietary calcium or vitamin D is insufficient, the parathyroid gland increases secretion of PTH to enhance calcium mobilization from bone to maintain serum calcium levels. Excessive hormone levels, whether transient or prolonged, can lead to abnormally elevated urine calcium (hypercalciuria), blood calcium (hypercalcemia) and blood phosphorus (hyperphosphatemia). Insufficient hormone levels can lead to the opposite syndrome of abnormally low blood calcium levels (hypocalcemia). Vitamin D hormones are also required for the normal functioning of the musculoskeletal, immune and renin-angiotensin systems. Numerous other roles for Vitamin D hormones are being postulated and elucidated, based on the documented presence of intracellular VDR in nearly every human tissue.
Left untreated, inadequate Vitamin D supply can cause serious bone disorders, including rickets and osteomalacia, and may contribute to the development of many other disorders including osteoporosis, non-traumatic fractures of the spine and hip, obesity, diabetes, muscle weakness, immune deficiencies, hypertension, psoriasis, and various cancers.
The Institute of Medicine (IOM) of the National Academy of Sciences has concluded that an Adequate Intake (AI) of Vitamin D for a healthy individual ranges from 200 to 600 IU per day, depending on the individual's age and sex (Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Dietary reference intakes: calcium, phosphorus, magnesium, vitamin D, and fluoride. Washington, D.C.: National Academy Press (1997), incorporated by reference). The AI for Vitamin D was defined primarily on the basis of a serum 25-hydroxyvitamin D level sufficient to prevent Vitamin D deficiency rickets or osteomalacia (or greater than or equal to 11 ng/mL). The IOM also established a Tolerable Upper Intake Level (UL) for Vitamin D of 2,000 IU per day, based on evidence that higher doses are associated with an increased risk of hypercalciuria, hypercalcemia and related sequelae, including cardiac arrhythmias, seizures, and generalized vascular and other soft-tissue calcification.
Currently available oral Vitamin D supplements are far from ideal for achieving and maintaining optimal blood 25-hydroxyvitamin D levels. These preparations typically contain 400 IU to 5,000 IU of Vitamin D3 or 50,000 IU of Vitamin D2 and are formulated for quick or immediate release in the gastrointestinal tract. When administered at chronically high doses, as is often required for Vitamin D repletion, these products have significant, and often severe, limitations.
Abnormalities of Vitamin D signaling and metabolism exist in a wide variety of tumors (Krishnan et al., (2012). Rheum Dis Clin North Am 38, 161-178) and are thought to be due to increased expression of CYP24 (Luo et al., (2013) J Steroid Biochem Mol Biol 136, 252-257). Cancer patients generally exhibit vitamin D insufficiency, therefore, calcium resorption from bone calcium stores plays a dominant role in the normalization of blood calcium levels. Regardless of the cancer type, low serum levels of 25-hydroxyvitamin D and decreased VDR activation have been associated with increased metastasis. Cancer mortality is usually a consequence of metastasis. For certain types of cancer, notably breast and prostate, the bulk of tumor burden at the time of death is in bone. The impact of metastasis on bone metabolism and consequent morbidity is considerable and, depending on the origin of the primary tumor, is either osteolytic (e.g., breast, myeloma) or osteoblastic (e.g., prostate) in nature. However, since bone formation and bone resorption are coupled, “osteolytic” and “osteoblastic” categorizations correspond to the net balance of bone metabolism associated with metastases. A number of factors released from tumors can affect net balance of bone metabolism, including parathyroid hormone related peptide (PTHrP), transforming growth factor-β (TGF-β), insulin-like growth factors (IGF), bone morphogenetic factors (BMP) and platelet-derived growth factors (PDGF).
PTHrP is produced by certain types of cancer cells, such as breast, and can trigger net bone resorption by stimulating the production of the ligand for the receptor activator of NFκB (RANKL) (Rabbani, S. A. (2000). Int J Oncol 16, 197-206.; Soyfoo et al. (2013). Support Care Cancer 21, 1415-1419). Like PTH, PTHrP can be regulated by activating the Vitamin D signaling pathway (Bhatia et al. (2009). Mol Cancer Ther 8, 1787-1798; El Abdaimi et al. (1999). Cancer Res 59, 3325-3328.). Consequently, the use of Vitamin D and related analogs has been proposed to help control excessive hypercalcemia caused by PTHrP overexpression in breast and prostate cancers (Richard et al. (2005) Crit Rev Eukaryot Gene Expr 15, 115-132.). The majority of instances of hypercalcemia in cancer patients are thought to be related to the production of PTHrP (Motellon et al. (2000) Clin Chim Acta 290, 189-197.). In some cases, hypercalcemia of malignancies has been associated with the use of Vitamin D or calcifediol and is related to elevated PTHrP expression. Like PTH, PTHrP expression can increase expression of CYP27B1, the kidney enzyme responsible for activating calcifediol. Therefore, a cancer patient with vitamin D insufficiency and higher than normal levels of PTHrP could potentially express increased levels of unoccupied CYP27B1; a sudden bolus of calcifediol could cause a surge in 1,25-dihydroxyvitamin D and potentially result in hypercalcemic episodes (Motellon et al 2000, supra; Sato et al. (1993). Intern Med 32, 886-890.) and further upregulation of CYP24. These hypercalcemic episodes, in contrast to those caused by PTHrP stimulation of RANKL, are due to increased rate of intestinal absorption of Ca.
The relationship between the progression of tumor metastases and bone catabolism is determined to a large extent on the tumor microenvironment within bone. In certain types of cancers, such as prostate cancer, bone formation can be stimulated by TGF-β, IGFs, PDGF and BMPs and these factors play an important role in establishing the bone microenvironment. These patients can suffer from hypocalcemia, which is the reduction of serum calcium levels in the blood. Severe hypocalcemia is sometimes referred to as “hungry bone” syndrome. Accordingly, the state of bone health may be an important determinant of the progression of the metastatic process, including the tumor cell invasion of bone, the angiogenic response, and tumor cell proliferation, as well as differentiation of bone cell precursors into osteoblasts and osteoclasts. There is evidence that vitamin D status may have an influence on each of these parameters, suggesting that vitamin D adequacy may be essential to minimize the progression of bone metastases. Although numerous clinical studies have attempted to raise Vitamin D levels for the treatment of various cancers, currently available therapies do not safely raise 25-hydroxyvitamin D levels high enough to establish the impact 25-hydroxyvitamin D has on tumor growth and metastasis or associated morbidities.
Because bone resorption is a common pathophysiology of bone metastases regardless of primary tumor type, patients are typically treated with bone antiresorptive agents, which inhibit bone resorption by targeting bone osteoclasts to decrease their osteolytic activity. Antiresorptive therapies, also known as bone-sparing agents, reduce the impact of cancer-related increases in bone resorption. Antiresorptive agents can prevent or delay skeletal related events (SRE). SRE are defined as pathological fractures, radiation or surgery to bone, and spinal cord compression, and are used to evaluate the clinical efficacy of antiresorptive agents because SRE are associated with poor prognosis and quality of life. Because antiresorptive agents can slow bone loss, they are also prescribed for patients with osteoporosis and other bone disorders. Examples of antiresorptive agents include bisphosphonates such as zoledronic acid, selective estrogen receptor modulators (SERMs), calcitonin, estrogen, and monoclonal antibodies such as denosumab. Treatment with antiresorptive agents also reduces the efficiency of PTH-stimulated resorption of bone, thus patients must rely on intestinal absorption of calcium for maintaining serum calcium levels.
One of the most important and immediate side effects of antiresorptive agents is hypocalcemia. Other therapeutic agents that can increase the risk of hypocalcemia include anticonvulsant agents, corticosteroids, antihypercalcemia agents, antimicrobial agents, and combinations thereof. Serum calcium is critical for the normal function of nerves and muscles in the body, and serum calcium levels are tightly regulated within narrow limits in healthy subjects. Hypocalcemia can be a significant source of morbidity and mortality. Severe hypocalcemia, in which serum calcium levels are reduced to below the lower limit of normal, can result in life-threatening consequences, including muscle tetany and cardiac arrest. Such treatment-induced, also known as iatrogenic, hypocalcemia, can be serious, even fatal, and therefore must be controlled.
Following administration of the antiresorptive agent denosumab, hypocalcemia is believed to result directly from the inhibitory effects of denosumab on the activity and numbers of bone-resorbing osteoclastic bone cells. Clinical studies have suggested reduced levels of calcium in the blood as soon as one day after initiation of denosumab treatment. Similarly, in a recent study of patients with bone metastases treated with the antiresorptive agent zoledronic acid, 39% of the patients developed hypocalcemia (Zuradelli et al., (2009) Oncologist 14, 548-556). Hypocalcemia is one of the most common adverse reactions resulting in discontinuation of therapy with zoledronic acid or denosumab.
Another example of a therapeutic agent that can increase the risk of hypocalcemia is the antihypercalcemia agent cinacalcet (SENSIPAR, Amgen Inc., Thousand Oaks, Calif.). Cinacalcet activates calcium-sensing receptors in the body and lowers serum calcium. See, e.g., U.S. Pat. Nos. 6,001,884 and 6,211,244, incorporated herein by reference. Cinacalcet is currently indicated for treating secondary hyperparathyroidism in patients having Chronic Kidney Disease (CKD) on dialysis (i.e., CKD Stage 5) and hypercalcemia in patients with parathyroid carcinoma or primary hyperparathyroidism. Cinacalcet may cause significant reductions in serum calcium that can lead to hypocalcemia and/or seizures and is contraindicated for use in patients who are already hypocalcemic and also is not indicated for use in CKD patients who are not on dialysis due to the increased risk of hypocalcemia. It is contemplated that the compositions and methods herein can be useful in patients having CKD Stage 5, or in another embodiment in patients having CKD Stage 4. It is contemplated that the compositions and methods herein can be useful in patients having CKD and on dialysis, or in another embodiment, patients not on dialysis.
Vitamin D supplementation is therefore recommended for patients on antiresorptive therapy and/or therapy including an agent that increases the risk of hypocalcemia such as cinacalcet. The treatment protocols in published repeat-dose clinical studies for denosumab have uniformly called for denosumab-treated subjects to receive daily supplements of calcium (0.5 to 1.0 g or more) and at least 400 to 800 IU vitamin D (cholecalciferol and/or ergocalciferol) in order to prevent hypocalcemia. Recommendations for calcium and vitamin D supplementation of denosumab-treated subjects have been included in the FDA-approved labeling for denosumab. However, currently available oral vitamin D supplements are not optimal for increasing and maintaining serum levels of either 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D at desirable levels. The inadequacy of currently available vitamin D supplements at completely mitigating hypocalcemia in denosumab-treated subjects is highlighted by a recent Advisory from Health Canada, which noted that postmarketing cases of severe symptomatic hypocalcemia have occurred in denosumab-treated subjects at an estimated rate of 1 to 2%, including some cases that were fatal.
Another side effect of antiresorptive agents and other agents that increase the risk of hypocalcemia is secondary hyperparathyroidism (SHPT). Decreases in serum calcium can result in increased production of PTH. Elevated PTH levels are common in patients undergoing treatment with antiresorptive agents, indicating an increased vitamin D requirement. Regulation of blood calcium requires adequate production of calcitriol, which stimulates intestinal absorption of dietary calcium and reabsorption of calcium by the kidney. Calcitriol, in concert with elevated PTH, also mobilizes calcium from bone. Adequate calcitriol production requires a sufficient supply of the precursor, calcifediol, and the first sign of inadequate calcitriol production is an increase in plasma PTH. PTH stimulates expression of CYP27B1 in the kidney and, thereby, increases conversion of calcifediol to calcitriol. When serum calcitriol levels are restored to adequate levels, PTH secretion decreases. If serum calcitriol levels cannot be corrected, as in the case of a calcifediol supply shortage (i.e., vitamin D insufficiency), plasma PTH remains elevated causing continuous mobilization of calcium from bone. A recent study (Berruti et al. (2012) Oncologist 17, 645-652) reported that 82% to 90% of subjects with prostate cancer metastatic to bone and receiving zoledronic acid exhibited elevated PTH, compared to 17% of patients receiving placebo. The elevated PTH was negatively associated with survival. The prevalence and persistence of SHPT in patients on antiresorptive therapies even though supplemented with Vitamin D and calcium indicates that appropriate supplementation regimens have not yet been clearly defined for this patient population, and the efficacy of antiresorptive agents can be limited by even mild hypocalcemia and/or SHPT.
Clearly, an alternative approach to currently available Vitamin D supplementation is needed in patients with cancer and in patients treated with an agent that increases the risk of hypocalcemia.